Many portable battery-operated devices include a rechargeable battery which operates at a voltage which is the same as that of the electrical system of many internal combustion engines. For example, many battery-operated devices include a rechargeable battery having a 12 volt rating, the same as that of the battery of most automotive vehicles. When charging such a battery from an automotive vehicle, it is generally not connected directly to the vehicle's electrical system in order to avoid possible damage to the device battery. Direct connection of the device battery to the vehicle's electrical system presents no serious damage problem to the device battery when the vehicle engine is running since charging of both the vehicle battery and the device battery is controlled by the vehicle voltage regulator. Typically, the charging voltage is kept above the voltage rating of the vehicle battery (and the device battery) and yet below a voltage which could damage the vehicle battery (and the device battery). When the vehicle engine is shut off, there is little danger of damaging the device battery, and if the vehicle battery is in reasonably good condition, the device battery can be fully or substantially fully charged.
With the device battery connected directly to the vehicle electrical system, it is possible, and also likely, however, to damage the device battery during starting of the vehicle engine. Since the voltage of the vehicle electrical system drops considerably during starting, and quite often to a value which is less than the voltage level of the device battery, in such a case, the device battery supplies current to help start the vehicle engine. With its relatively low capacity, it is likely in that siutation that the device battery will be damaged, particularly where there is a large voltage difference between the voltage level of the device battery and that of the vehicle's electrical system during starting.
The aforementioned problem can be corrected by putting a rectifier diode in series with the vehicle battery and the device battery. The series diode prevents discharging of the device battery to the vehicle battery when the vehicle battery voltage is less that that of the device battery, which occurs during starting of the vehicle engine. When the vehicle engine is running and the vehicle battery charging system is operating, the voltage at the vehicle generator typically exceeds the voltage of the device battery plus the voltage drop in the diode. The diode being forward-biased allows charging of the device battery. However, when the vehicle engine is not running, charging of the device may not continue from the vehicle battery because of the voltage drop in the diode. Hence, if the device battery does not reach its full potential with the engine running, the device battery will not reach the full voltage of the vehicle battery or its own full power with the engine off. Thus, while it is possible to charge the device battery from the electrical system of an automotive vehicle having the same voltage rating as the device battery, in many instances it is not possible to fully charge the device battery and at the same time safeguard the device battery from damage during starting.